Among the choices you'll have one called 'Group' and to change plugin load order just choose another group from the drop-down list of groups. To change the current group of a plugin, double-click a plugin you want to change the current order of to open an extra dialogue on the right. In the Group column you'll see the current group for all plugins, the most common would be the Default group.

First go to Vortex plugins-tab, if you don't currently see the 'Group'-column click on the gear-icon on the far right and enable more columns.

For plugin load order, the easiest and fastest method in Vortex is to sort by Groups.